The natural heat of the earth is a vast energy resource estimated to be nearly 300 times larger than that obtainable from the combustion of all known reserves of fossil fuels combined. This resource, known as geothermal energy, flows as a continuous heat flux from the molten core through the mantle and crust to the surface of the earth. The temperature gradient of the geothermal heat flux averages 25.degree. C. per kilometer of depth in the crust over most of the earth's surface. Higher gradients, e.g., 100.degree. C./km, exist in parts of the world having geologically favorable structure and history. Still higher gradients are found in volcanic regions near tectonic boundaries.
Two principal methods are presently employed in conventional geothermal power generation. One method involves the use of natural steam obtained from a geothermal reservoir and expanded in steam turbines adapted from conventional technology for geothermal use. The other method involves the transfer of heat from the geothermal fluid to a working fluid, which is expanded in turbines made for use with the specific working fluid. The latter method is commonly referred to as a binary cycle, and is distinguished from the former method which is commonly referred to as a steam cycle. Combinations of the steam and binary cycles are sometimes employed to further utilize the available energy of the geothermal resource. Such plants are commonly referred to as geothermal combined cycle, or alternatively, as hybrid cycle power plants.
Steam for geothermal power generation may be produced directly, as is the case where the geothermal resource is manifested as a reservoir of dry steam, or it may be derived from a separation process where the reservoir is of the hot water, or hydrothermal, type. In hydrothermal reservoirs, a two-phase mixture of steam and hot water are generally produced from wells and then separated at the surface. The separated steam can be expanded in a steam turbine and the separated water can provide heat for a binary cycle power plant. Alternatively, a hydrothermal reservoir may be pumped in which case it is possible to bring pressurized hot water to the surface. The hot water can be used in a binary cycle or it may be flashed to produce steam for use in a steam cycle. A third type of geothermal resource, which is perhaps the most widespread but has not been developed commercially, is the hot dry rock which exists at some depth everywhere in the world. To extract heat from hot dry rock, a fluid must be circulated from the surface down to the rock formation and back again for the purpose of conveying the underground heat to the surface. Water is the fluid most commonly contemplated for such purposes, but other fluids with suitable properties might be used.
There have also been attempts to devise mechanical means to directly utilize the total flow of fluid from the geothermal reservoir without first resorting to a separation process or heat exchangers. These machines are designed to accept both the liquid and vapor fractions of a two-phase geothermal fluid, separate and expand them, and produce useful work in the process. Utilization of the total flow promises greater energy conversion, however, these machines have not proven commercially successful. Geothermal MHD generators are also total flow devices, but since they do not rely on moving parts in the geothermal process stream, they do not suffer the same disadvantages as do their mechanical counterparts.
Magnetohydrodynamic generators are similar to conventional generators in that a voltage is induced in a conductor as a result of the relative motion between the conductor and a magnetic field, in accordance with Faraday's Law. However, in a conventional generator the conductor is a solid metal such as copper, while in a MHD generator it is an electrically conductive fluid. MHD generators have been described using liquid metal, high temperature plasma, or even seawater as the electrically conductive fluid.
The electrical conductivity of the fluid is a key variable in the performance of MHD generators. The conductivity of fluids commonly considered varies over several orders of magnitude. The conductivity of seawater is approximately 4 Siemens per meter; seeded plasmas range from 1 to 10.sup.4 S/m; liquid metals range from 10.sup.5 to 10.sup.7 S/m. The conductivity of geothermal fluids also varies widely, ranging from 10.sup.-1 to 10.sup.2 S/m.